From Ed Tomlinson, then mauled by yours truly.

The current shrinking of the dentry, inode and dquot caches seems to
work OK, but it is slightly CPU-inefficient: we call the shrinking
functions many times, for tiny numbers of objects.

So here, we just batch that up - shrinking happens at the same rate but
we perform it in larger units of work.

To do this, we need a way of knowing how many objects are currently in
use by individual caches.  slab does not actually track this
information, but the existing shrinkable caches do have this on hand.
So rather than adding the counters to slab, we require that the
shrinker callback functions keep their own count - we query that via
the callback.

We add a simple registration API which is exported to modules.  A
subsystem may register its own callback function via set_shrinker().

set_shrinker() simply takes a function pointer.  The function is called
with

	int (*shrinker)(int nr_to_shrink, unsigned int gfp_mask);

The shrinker callback must scan `nr_to_scan' objects and free all
freeable scanned objects.  Note: it doesn't have to *free* `nr_to_scan'
objects.  It need only scan that many.  Which is a fairly pedantic
detail, really.

The shrinker callback must return the number of objects which are in
its cache at the end of the scanning attempt.  It will be called with
nr_to_scan == 0 when we're just querying the cache size.

The set_shrinker() registration API is passed a hint as to how many
disk seeks a single cache object is worth.  Everything uses "2" at
present.

I saw no need to add the traditional `here is my void *data' to the
registration/callback.  Because there is a one-to-one relationship
between caches and their shrinkers.


Various cleanups became possible:

- shrink_icache_memory() is no longer exported to modules.

- shrink_icache_memory() is now static to fs/inode.c

- prune_icache() is now static to fs/inode.c, and made inline (single caller)

- shrink_dcache_memory() is made static to fs/dcache.c

- prune_dcache() is no longer exported to modules

- prune_dcache() is made static to fs/dcache.c

- shrink_dqcache_memory() is made static to fs/dquot.c

- All the quota init code has been moved from fs/dcache.c into fs/dquot.c

- All modifications to inodes_stat.nr_inodes are now inside
  inode_lock - the dispose_list one was racy.